Magnetic properties of the frustrated antiferromagnetic spinel ZnCr2O4 and the spin-glass Zn1-xCdxCr2O4 (x=0.05,0.10) -: art. no. 024408

The T dependence (2-400 K) of the electron paramagnetic resonance (EPR). magnetic susceptibility chi (T), and specific heat C-v(T) of the normal antiferromagnetic (AFM) spinel ZnCr2O4 and the spin-glass (SG) Zn1-xCdxCr2O4 (x = 0.05.0.10) are reported. These systems behave as a strongly frustrated AFM and SG with T(N)approximate toT(G)approximate to 12 K and -400 K greater than or similar to Theta (CW)greater than or similar to -500 K. At high-ir the EPR intensity follows the chi (T) and the g value is T independent. The linewidth broadens as the temperature is lowered, suggesting the existence of short range AFM correlations in the paramagnetic phase. For ZnCr2O4 the EPR intensity and chi (T) decreases below 90 and 50 K, respectively. These results are discussed in terms of both nearest-neighbor Cr3+ (S=3/2) spin-coupled pairs and spin-coupled tetrahedral clusters with an exchange coupling of \J/k\ approximate to 35-45 K. The appearance of small resonance modes for T less than or similar to 17 K. the observation of a sharp drop in chi (T) and a strong peak in C-v(T) at T-N=12 K confirms, as previously reported, the existence of long range AFM correlations in the low-T phase. A comparison with recent neutron diffraction experiments, that found a near dispersionless excitation at 4.5 meV for T less than or similar toT(N) and a continuous gapless spectrum for T greater than or similar toT(N), is also given.